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Associated Press
Three portraits of Albert Einstein were taken in Princeton, USA, in March 1953. Einstein was born at Ulm, Germany on March 14, 1879.

Theoretical physicist Albert Einstein — born in Southern Germany on this day in 1879 — has become a universal icon of intelligence and scientific discovery over the nearly 150 years since his birth.

His theories revolutionized the way we see the universe, altering our perception of space and time and introducing the scientific basis for nuclear energy. Einstein received the 1921 Nobel Prize in Physics for “his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."

In 1999, Time magazine named Einstein Person of the Century, declaring him “the pre-eminent scientist in a century dominated by science.” To many, “Einstein” is now synonymous with “genius.”

Known for his playful humor, Einstein is also celebrated for his many pithy aphorisms.

Associated Press
Dr. Albert Einstein writes out an equation for the density of the Milky Way on the blackboard at the Carnegie Institute, Mt. Wilson Observatory headquarters in Pasadena, Calif., on Jan. 14, 1931.

In October 2017, a piece of paper on which Einstein wrote, "A calm and modest life brings more happiness than the pursuit of success combined with constant restlessness," sold for $1.3 million at auction.

According to NPR’s Morning Edition, Einstein penned the sentence in 1922 after learning he had just won the Nobel Prize. At the time, he didn’t have money to tip a bellboy, so he offered him this scrap of advice instead.

In honor of Einstein's birthday, here are 21 more quotes that give us a glimpse into his wit, humanity and deep curiosity.

On authority:

"Unthinking respect for authority is the greatest enemy of truth."

— 1901 letter to Swiss linguist Jost Winteler

On being present:

"A happy man is too satisfied with the present to think too much about the future."

— 1896 school essay titled "My Future Plans," written at age 17

On certainty:

"As far as the propositions of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality."

— 1921 lecture before the Prussian Academy of Sciences

Princeton University via Associated Press
Dr. Albert Einstein is photographed in his office in Berlin in the early 1920s.

On common sense:

"Common sense is nothing more than a deposit of prejudices laid down in the mind before you reach eighteen."

Attributed to Einstein by Lincoln Barnett in the 1948 book "The Universe and Dr. Einstein"

On curiosity:

"I have no special talents. I am only passionately curious."

— 1952 letter to Einstein’s first biographer, Carl Seelig

On death:

"Our death is not an end if we have lived on in our children and the younger generation. For they are us; our bodies are only wilted leaves on the tree of life."

— 1926 letter to Maria Bijleveld, the widow of Dutch physicist Heike Kamerlingh Onnes

On education:

"The aim (of education) must be the training of independently thinking and acting individuals, who, however, see in the service of the community their highest life problem."

— 1931 address at the State University of New York at Albany

Associated Press
This undated photo shows legendary physicist Dr. Albert Einstein, author of the theory of relativity.

On fame:

"With fame I become more and more stupid, which of course is a very common phenomenon."

— 1919 letter to physiology professor Heinrich Zangger

On his own genius:

"It strikes me as unfair, and even in bad taste, to select a few individuals for boundless admiration, attributing superhuman powers of mind and character to them. This has been my fate, and the contrast between the popular assessment of my powers and achievements and the reality is simply grotesque."

Quote from a 1921 interview in Nieuwe Rotterdamsche Courant, a Dutch newspaper

On his physical appearance:

"Let me tell you what I look like: pale face, long hair, and a tiny start of a paunch. In addition, an awkward gait, and a cigar in the mouth ... and a pen in pocket or hand. But crooked legs and warts he does not have, and so is quite handsome — also there's no hair on his hands, as is so often the case with ugly men. So it really is a pity that you didn't see me."

— 1920 postcard to eight-year-old cousin Elisabeth Ney

On imagination:

"I am enough of the artist to draw freely upon my imagination. Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world."

Quote from a 1929 interview published in The Saturday Evening Post

Associated Press
Famed physicist Albert Einstein is shown playing the violin, date and location unknown.

On the limits of human knowledge:

"As a human being, one has been endowed with just enough intelligence to be able to see clearly how utterly inadequate that intelligence is when confronted with what exists."

— 1932 letter to Queen Elisabeth of Belgium

On moving forward:

"People are like bicycles. They can keep their balance only as long as they keep moving."

— 1930 letter to Einstein’s son, Eduard

On music:

"If… I were not a physicist, I would probably be a musician. I often think in music. I live my daydreams in music. I see my life in terms of music. I cannot tell if I would have done any creative work of importance in music, but I do know that I get most joy in life out of my violin."

Quote from a 1929 interview published in the Saturday Evening Post

On mystery:

"The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. He who knows it not and can no longer wonder, no longer feel amazement, is as good as dead, a snuffed-out candle."

— "The World As I See It," 1949

Associated Press
Professor Albert Einstein and his wife Elsa arrive from Palestine to raise funds for Zionism on April 4, 1921.

On national identity:

"I am by heritage a Jew, by citizenship a Swiss, and by disposition a human being, and only a human being, without any special attachment to any state or national entity whatsoever."

— 1918 letter to German mathematician Adolf Kneser

On nationalism:

"Nationalism is an infantile disease. It is the measles of mankind."

Quote from a 1929 interview published in The Saturday Evening Post

On relativity:

"When a man sits with a pretty girl for an hour, it seems like a minute. But let him sit on a hot stove for a minute — and it's longer than any hour. That's relativity."

Quote from James B. Simpson's book "Best Quotes of '54, '55, '56"

On science:

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"One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike — and yet it is the most precious thing we have."

Quote from Banesh Hoffmann’s 1972 "Albert Einstein: Creator and Rebel"

On success:

"If A is a success in life, then A equals X plus Y plus Z. Work is X; Y is play; and Z is keeping your mouth shut."

Quote from a 1950 Observer article

On worry:

"I never think of the future. It comes soon enough."

Attributed to a December 1930 interview on the Belgenland, a ship on which Einstein traveled to New York that month